1. Field of the Invention
The invention relates to a process for integrated utilization of the energy and material contents of hydrolysates and solids obtained in the enzymatic hydrolysis of renewable raw materials, in which the resulting hydrolysis solution is used as a carbon source in fermentations and the unhydrolysed solids are sent to biogas production.
2. Background of the Invention
Fermentative processes for preparing target substances, for example amino acids, vitamins and carotenoids, by means of microorganisms are common knowledge. Depending on the different process conditions, different carbon sources are utilized. These range from pure sucrose through raw molasses from beet and sugar, so-called “high-test molasses” (inverted sugar molasses), up to and including glucose from starch hydrolysates. For the biotechnology production of L-lysine, acetic acid and ethanol are additionally mentioned as cosubstrates usable on the industrial scale (Pfefferle et al., Biotechnogical Manufacture of Lysine, Advances in Biochemical Engineering/Biotechnology, Vol. 79 (2003), 59-112).
An important carbon source for the fermentation of microorganisms is starch. This first has to be liquefied and saccharified in preceding reaction steps before it can be utilized as a carbon source in a fermentation. To this end, the starch is obtained from a natural starch source such as potatoes, cassava, cereal, e.g. wheat, maize, barley, rye or rice, typically in prepurified form, then enzymatically liquefied and saccharified in order then to be used in the actual fermentation to produce the target substances.
More recent techniques are concerned with improved methods which are intended to enable the production of fermentation media from renewable resources (EP 1205557, US 2002/079268).
A fermentative process has also already been described, with which it is said to be possible to use starch as a carbon source (WO 2005116228).
A fermentation does not only form the desired product, but always biomass too. This biomass is either disposed of as a waste product or has to be utilized in another way and reduces the yield (product per reactant) of the process by virtue of its formation. In fermentative ethanol preparation, complex animal feeds are therefore often produced as coproducts. In the so-called dry-milling process, by which about 65% of the ethanol is prepared, nearly four tons of DDGS (distillers dried grains with solubles) are produced in the United States of America alone (Lyons 2003, Jacques 2003). In summary, the so-called dry-milling process can be described as follows: the cereal grains are ground to fine particles in a mill and mixed with liquid. This slurry is then treated with a liquefying enzyme in order to hydrolyse the cereal to dextrins, which are a mixture of oligosaccharides. The hydrolysis of starch with the liquefying enzyme, known as α-amylase, is carried out above the gelation temperature of the cereal. The slurry is boiled at an appropriate temperature to break up the granular structure of the starch and to trigger the gelation. Finally, the dextrins formed are hydrolysed further to glucose with the exoenzyme glucoamylase in a saccharization process. The DDGS obtained is the main coproduct in ethanol production. Approximately 80% of this DDGS is fed to ruminants. This means that the utilization is only economically viable if enough ruminant breeding operations are present in the vicinity of the production plant as recipients for the DDGS.
This utilization of by-products of fermentative ethanol preparation as complex animal feed must, however, be distinguished from the preparation of target substances suitable as animal feed additives. In these processes, for example, amino acids or vitamins are produced by fermentation as main products and find use in animal nutrition. In the fermentation process, complex by-products are additionally obtained, which comprise the biomass. One means of utilizing the by-products is the production of fertilizers from the fermentation broth (Ideka 2003). For lysine preparation, processes in which the product is not purified after the fermentation but rather the biomass is also sold as a constituent of the animal feed additive are also used (Biolys® U.S. Pat. No. 5,431,933). As a further idea for utilization of the biomass obtained, biomass recycling has been published (Blaesen et al. 2005). In this process, the average yield can be increased and the amount of waste to be disposed off can be reduced in fermentation processes by recycling the biomass obtained as reactants into the fermentation.
The prior art discloses the anaerobic degradation of organic substances by bacteria to form biogas which consists of methane to an extent of 50-85%. The energy stored in biogas and obtained is referred to as renewable in that it stems from renewable organic substance. In addition, the energetic utilization of biogas, in contradistinction to the combustion of natural gas, mineral oil or coal, is carbon dioxide-neutral because the carbon dioxide which forms moves within the natural carbon cycle and is consumed again by the plants during their growth.
Biogas is a high-value energy source, i.e. it can be utilized in many ways and with high efficiency. The main intake source in biogas production is currently the yield from power generation. By means of the so-called power-heat coupling, the biogas is used in a classical manner as a fuel in an internal combustion engine which drives a generator for generating mains current (alternating current). The waste engine heat from cooling system and exhaust gas which is obtained simultaneously can be utilized for heating. As an alternative to power generation with high efficiency, fuel cells are also already being used. Biogas can also be produced from wastes of L-lysine production (Viesturs et al. 1987 Proc. Latv. Acad. Sci 8 (841):102-105). A further process is said, in an integrated process, to enable the parallel production of meat (or milk), ethanol, animal feed and biogas (biofertilizer) (US 2005/0153410). In the case of use of protein-containing coreactants in anaerobic fermentation processes, difficulties can be encountered, since the proteins, in the event of changes in the pH in the presence of divalent cations, can undergo structural changes which can prevent a later enzymatic attack (Mulder 2003, Biological wastewater treatment for industrial effluents; technology and operation, Paques B. V., Balk 3.1 Fermentation).
The typical degradation process of organic material to biogas consists essentially of four stages.
In the first stage (hydrolysis), aerobic bacteria convert the high molecular weight organic substances (protein, carbohydrates, fat, cellulose) with the aid of enzymes to low molecular weight compounds such as simple sugars, amino acids, fatty acids and water. The enzymes excreted by the hydrolytic bacteria adhere to the outside of the bacteria (so-called exoenzymes) and split the organic constituents of the substrate hydrolytically into small water-soluble molecules. In the second stage (acidification), the individual molecules are degraded and converted intracellularly by acid-forming bacteria. These are possibly aerobic species which consume the oxygen still remaining and thus provide the anaerobic conditions needed for the methane bacteria. Here, mainly short-chain fatty acids, low molecular weight alcohols and gases are obtained. In the third stage (acetic acid formation), acetic acid bacteria produce the starting materials for the methane formation (acetic acid, carbon dioxide and hydrogen) from the organic acid. In the fourth stage (methane formation), methane bacteria form the methane (Eder and Schulz 2006).
The costs for the provision of the carbon source, just like the energy costs, have a considerable influence on the margins in the fine chemicals business. The most significant item in the preparation costs of, for example, L-lysine is the carbon source (Pfefferle et al. 2003). The price of sugar is subject to high variations and has a major influence on the economic viability of the fermentation processes, especially in the case of the low-cost mass products such as monosodium glutamate, L-lysine-HCl and L-threonine, whose market is determined greatly by the competition (Ikeda 2003). The provision of carbon sources suitable for fermentative utilization from favourable renewable raw materials is, however, not trivial in industry. In the workup of these carbon sources by hydrolysis, fibrous plant residues are obtained as waste products to be disposed of. In addition, biomass forms in the fermentation as a waste product to be disposed of. When biogas production is selected as a disposal process, difficulties can be encountered in the use of protein-containing coreactants in the anaerobic fermentation process used. In the event of changes in the pH in the presence of divalent cations, the proteins can undergo structural changes which can prevent later enzymatic attack (Mulder 2003). Variations in quality of the raw materials additionally hinder a stable and reproducible process regime in the biogas production.